Method of coating metal on dielectric material



Feb. 6, 1962 w. E. ROWE 3,020,155

METHOD OF COATING METAL ON DIELECTRIC MATERIAL Original Filed May 10, 1957 FIG. 3.

DIELECTRIC MATERIAL S'LVER COAT'NG LIGHT L16 g l NEGATIVE t v i v I2 7 2o I0 V FIG. 2.

T T, I4 Iv '2 FIG. 4.

V m V V '4 IOOOO m PREFERRED g 50 D Z 2 E u l0 2 INVENTOR TEMPERATURE IN DEGREES WME ROI! FAHRENHEIT BY 2 ATTORNEYS.

, 3,020,156.- METHOD OF COATING METAL N DIELECTRIQ MATERIAL William E. Rowe, Newburyport, Mass., assignor to Mycalex Corporation of America, Clifton, NJ., a corporation of New York Continuation of application Ser. No. 658,390, May 10, 1957. This application Apr. 4, 1960, Ser. No. 19,595 4 Claims. (Cl. 96-35) This invention relates to a method of coating metal on dielectric material and particularly to a method for coating metal on preselected portions of a piece of dielectric material.

This application is a continuation of my earlier appli cation Serial Number 658,390, filed by me on May 10, 1957, and assigned to the assignee hereof, now abancloned.

The coating of metals on dielectric materials has recently become an extremely important art in view of the growth of and interest in printed circuits and other items. Among the methods which have been used to coat metal on dielectric material has been a photoetching process well known in the art. However, no method heretofore produced has yielded a precise sharp image in metal. The sharpness of the image is often of extreme importance.

The main object of the present invention is to provide a new and improved method of coating metal on dielectric materials.

Another object of the present invention is to provide a new and improved method for photoetching metallic coatings on dielectric materials whereby to yield very sharp delineations along the edges of the coating.

The above and other objects, characteristics and features of the present invention will be more fully understood from the following description taken in connection with the accompanying illustrative drawing.

In the drawing:

FIG. 1 is a diagrammatic sectional view showing a dielectric material coated metal which forms one step in my novel method;

FIG. 2 is a view similar to FIG. 1 showing a resist overlying the metallic coating on the dielectric material, which condition illustrates another step in the present 1nvention;

FIG. 3 is a view similar to FIGS. 1 and 2 showing the exposure of the resist to light, which represents another step in the present method;

FIG. 4 is a diagrammatic view similar to FIGS. 1 to 3 showing the condition of the exposed resist after developing thereof, which illustrates another step in my novel method;

FIG. 5 is a diagrammatic view similar to FIGS. 1 t0 4 illustrating the actual removal of metal whereby to leave a metallic coating of desired configuration; and

FIG. 6 is a diagrammatic view similar to FIGS. 1 to 5 showing the final product;

FIG. 7 is a graph showing the time temperature relationship for baking the metallic coating and for baking the resist.

Referring now to the drawings in detail, any suitable dielectric material may be employed in my novel method. For instance, the dielectric material may be a vitreomicaceous material such as glass-bonded mica and ceramoplastic. In the alternative, the dielectric material may be made of glass, ceramic or some organic material. The requirements of the dielectric material will be more fully explained as my novel method is explained in detail. The dielectric material is designated generally by the reference numeral 10.

The first step in the present method is to coat a surface of dielectric material 10 with a metal. Preferably, this coating may be applied in the form of liquid metal" which is a suspension of metal in a suitable suspending liquid. Preferably, the metal is a good conductor such as silver although other metals may be employed. The step of coating the liquid metal on the dielectric material forms no novel part of my method and is fully described and claimed in US. Patent No. 2,752,237, granted to Oliver A. Short on July 26,1956, for Silver Powder and Method for Producing Same. Accordingly, the application of the silver coating and its composition will not be described in detail as these details can be obtained from a perusal of the aforementioned Short patent. Sutfice it to say the liquid silver comprises powdered silver suspended in a liquid such as a soda ash solution. The suspension is applied to the entire surface 12 of the dielectric material 10 and after it is so applied the coated dielectric material is placed in an oven for baking. The temperature and time of the baking of the silver suspension in order to drive ofl. the suspending liquid and to leave a coating of silver is dependent upon a time-temperature relation. The table below sets forth the time-temperature relationship which will yield a good silver coating on the dielectric material. FIG. 7 graphically' depicts the time-temperature relation for baking liquid silver of the type described in the aforementioned Short patent. If other types of metallic coatings are used, other time-temperature relationships may exist. The preferred time for any of the given temperatures is roughly half-way between the minimum and maximum times stated in said table.

The preferred time and temperature for baking the given liquid silver is about 400 F. for thirty minutes.

After the baking is completed there is a silver coating 14 which overlies surface 12 of dielectric material 10 and which adheres thereto with considerable strength. It is not my present aim to coat one entire surface of a dielectric material with metal. It is my aim to only coat portions of surface 12 with coating 14. Accordingly, havingfirst coated the entire surface 12 with coating 14, it-becomes necessary to remove undesired portions of the silver coating 14, while permitting the desired portions to remain in overlyingrela-tionship with dielectric material 10. To accomplish this aim, a light sensitive solution is coated over the upper surface 16 of silver coating 14. The resist formed by the light sensitive solution must be of a type that when heated forms a stronger bond with the upper surface 16 of the metal coating 14 than the metal coating 14 forms with the upper surface 12 of dielectric material 10. Moreover, and equally important, the resist must be shrinkable when heated. The following formulations yield resists which are suitable for use in my method.

Formulation I:

Concentrated ammonia drops .10

resist 22 will shrink and tend to flake off.

Formulation II:

Gum arabic (14 Baum) cc 1,000 Ammonium dichromate (20% solution) ..cc 340 Ammonia (28% solution), cc 50 Water-sufficient to. make solution with 14 Baum specific gravity.

Both of [the formulations described above will give satisfactory results when employed in the method being described and both work with a water developer as will be described hereinafter. The preferred formulation for the resist may be tabulated as-- Formulation III:

After the light sensitive material is sprayed onto the silver coating the light sensitive solution is subjected to heat of about 100 F. for a sufficient time to cause it to dry. Preferably, the heat is that emanated from an infrared lamp whereby to prevent exposure of the ammonium dichromate which is light sensitive.

Once the light sensitive coating has been dried, a negative is placed in overlying relationship therewith. The negative may be in the form of a cutout pattern but preferably it is in the form of a photographic plate having opaque and transparent portions to prevent and permit the transmission of light therethrough, respec! tively. The negative is designated by the reference nu metal 20. Once the negative is placed over the light sensitive material, a bright light is directed onto the negative and a portion of this light passes through the negative to expose the light sensitive material 18 and another portion is prevented from passing through due to the opaque parts of the negative. Accordingly, only those portions of the light sensitive material 18 underlying the transparent part of the negative 20 will be exposed.

After exposure the negative is removed and the light sensitive material is subjected to a developer. The composition of the developer will depend upon the type of light sensitive coating employed and the particular developers usable with the light sensitive formulations disclosed hereinbefore have already been described. The developer washes away the unexposed portions of the light sensitive material and leaves unafiected the exposed portions thereof. This condition is shown in FIG. 4. The developed but unremoved portions of the light sensitive material are commonly called a resist and are herein designated by the reference numeral 22.

After the formulation of the resist the entire resulting piece comprising the dielectric member 10, the silver coating 14 and the resist 22 is placed in an oven and is baked. It has been discovered that the time-temperature relationship used for baking the liquid silver is also suitable for baking the resists resulting from the light sensitive solution formulations disclosed hereinbefore. As was pointed out, any resist formed as a result of the use of the formulations described hereinbefore will shrink when heated. Moreover, and as is necessary to the working of my invention, the resist must bond to the silver with a stronger bond than the silver bonds to the dielectric. When these conditions are met and the resist is baked in accordance with the relationship described hereiubefore and preferably at about 400 F. for thirty minutes, the

In shrinkn however, the resist will remain firmly bonded to the silver underlying it and will pull the silver ofi of the underlying dielectric material. At the completion of the baking the flaked oif resist and silver may merely be brushed away or washed away or blown away and the result will yield a silver coating of desired pattern overlying the dielectric material 10 as is shown in FIG. 6.

I have found that the above described method not only is easy to work and costs very little, but additionally it yields very sharp silver images heretofore not readily obtainable by previously disclosed photoetching methods. Extremely intricate patterns of metal may thus be coated on dielectric materials with great case.

While I have herein shown and described the preferred form of my invention and have suggested various changes and modifications therein, other changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of this invention.

Having now described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. The method of coating a metal on a predetermined portion of the surface of a dielectric material, comprising the steps of coating said surface with a liquid silver suspension, baking said suspension at about 400 F. for about thirty minutes to drive off the liquid from said suspension and to cause said silver to bond to said surface, coating said silver with a light sensitive heat shrinkable solution consisting essentially of 16 ounces of commercial fish glue, 3 ounces of a 20% solution of ammonium dichromate, 10 drops of concentrated ammonia and 1 gallon of water, baking said solution at about F. until it dries, disposing a negative over said dried solution, illuminating said negative to expose a portion of said light sensitive material, developing said light sensitive material in water to wash away the exposed portion thereof, and baking the remaining portion of said light sensitive material at about 400 F. for about thirty minutes to cause said material to shrink and pull the underlying silver off said surface of said dielectric material.

2. The method of coating a metal on a predetermined portion of the surface of a dielectric material, comprising the steps of coating said surface with a liquid silver suspension, baking said suspension at about 400 F; for about thirty minutes to drive off the liquid from said suspension and to cause said silver to bond to said surface, coating said silver with a light sensitive heat shrinkable solution consisting essentially of 10 /2 ounces of fish glue, 2 ounces of egg White, 4 ounces of a 20% solution of ammonium dichrornate, 10 drops of concentrated ammonia and 16 ounces of water, baking said solution at about 100 F. until it dries, disposing a negative over said dried solution, illuminating said negative to expose a portion of said light sensitive material, developing said light sensitive material in water to wash away the exposed portions thereof, and baking the remaining portions of said light sensitive material at about 400 F. for about thirty minutes to cause said material to shrink and pull the underlying silver off said surface of said dielectric material.

3. The method of coating a metal on a predetermined portion of the surface of a dielectric material, comprising the steps of coating said surface with a liquid silver suspension, baking said suspension at about 400 F. for about thirty minutes to drive off the liquid from said suspension and to cause said silver to bond to said surface, coating said silver with a light sensitive heat shrinkable solution consisting essentially of 10 /2 ounces of fish glue, 2 /2 ounces of dried albumin, 4 ounces of a 20% solution of ammonium dichromate, 10 drops of concentrated ammonia and 16 ounces of water, baking said solution at about 100 F. until it dries, disposing a negative over said dried solution, illuminating said negative to expose a portion of said light sensitive material, developing said light sensitive material in water to wash away the exposed portion thereof, and baking the remaining portion of said light sensitive material at about 400 F. for about thirty minutes to cause said material to shrink and pull the underlying silver ofi said surface of said dielectric material.

4. The method of coating a metal on a predetermined portion of the surface of a dielectric material, comprising the steps of coating said surface with a liquid silver suspension, baking said suspension at about 400 F. for about thirty minutes to drive ofi the liquid from said suspension and to cause said silver to bond to said surface, coating said silver with a light sensitive heat shrinkable solution consisting essentially of 1,000 cc. of gum arabic having a specific gravity of about 14 Baum, 340 cc. of a 20% solution of ammonium dichromate, 50 cc. of a 28% solution of ammonia, and enough Water to yield a solution having a specific gravity of 14 Baum, baking said solution at about 100 F. until it dries, disposing a negative over said dried solution, illuminating said negative to expose a portion of said light sensitive material, developing said light sensitive material in a solution consisting essentially of 1,000 cc. of a calcium chloride solution having a specific gravity of 40 Baum and 53 cc. of an 85% solution of lactic acid to wash away the exposed portion thereof, and baking the remaining portion of said light sensitive material at about 400 F. for about thirty minutes to cause said material to shrink and pull the underlying silver off said surface of said dielectric material.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF COATING A METAL ON A PREDETERMINED PORTION OF THE SURFCE OF A DIELECTRIC MATERIAL, COMPRISING THE STEPS OF COATING SAID SURFACE WITH A LIQUID SILVER SUSPENSION, BAKING SAID SUSPENSION AT ABOUT 400*F. FOR ABOUT THIRTY MINUTES TO DRIVE OFF THE LIQUID FROM SAID SUSPENSION AND TO CAUSE SAID SILVER TO BOND TO SAID SURFACE, COATING SAID SILVER WITH A LIGHT SENSITIVE HEAT SHRINK ABLE SOLUTION CONSISTING ESSENTIALLY OF 16 OUNCES OF COMMERCIAL FISH GLUE, 3 OUNCES OF A 20, SOLUTION OF AMMONIUM DICHROMATE- 10 DROPS OF CONCENTRATED AMMONIA AND 1 GALON OF WATER, BAKING SAID SOLUTION AT ABOUT 100*F. UNTIL IT DRIES,DISPOSING A NEGATIVE OVER SAID DRIED SOLUTION, ILLUMINATING SAID NEGATIVE TO EXPOSE A PORTION OF SAID LIGHT SENSITIVE MATERIAL,DEVELOPING SAID LIGHT SENSITIVE MATERIAL IN WATER TO WASH AWAY THE EXPOSED PORTION THEREOF, AND BAKING THE REMAINING PORTION OF SAID LIGHT SENSITIVE MATERIAL AT ABOUT 400*F. FOR ABOUT THIRTY MINUTES TO CAUSE SAID MATERIAL TO SHRINK AND PULL THE UNDERLYING SILVER OFF SAID SURFACE OF SAID DIELECTRIC MATERIAL. 